High thermal stability of Si-containing Al-Zn-Mg-Cu crossover alloy caused by metastable GPB-II phase

Abstract

In this investigation, a Si-containing Al-Zn-Mg-Cu alloy with good thermal stability caused by metastable GPB-II phases was found. This metastable GPB-II phase will finally transform into the L phase at 175 °C. In comparison to the Si-free alloy that underwent peak aging at 175 °C, the tensile strength, yield strength and elongation of the alloy with the addition of 0.35 wt% Si exhibited an increase of 75 MPa (24.9 %), 88 MPa (37.8 %) and 3.1 % (41.8 %), respectively. Furthermore, the yield strength and microhardness of the alloy with a Si content exceeding 0.35 wt% exhibited minimal decline during over-aging at 175 °C for 96 h and 160 °C for 500 h. It was observed that the number density of the η and T phases at the peak aging state decreased, and the average size doubled when 0.2 wt% Si was added. And with further addition of Si, the η and T phases are no longer present within the alloy, with only the more stable, fine, and uniform GPB-II phase remaining. Transmission electron microscope (TEM) observations indicate that the average size of the GPB-II phase remains unchanged with prolonged aging at 175 °C, which provides a rationale for the alloy's well thermal stability. From the perspective of cluster formation, we put forth a three-stage growth model to explain the formation and evolution of the GPB-II core-shell structure. In comparison, the thermal stability of the GPB-II phase is markedly superior to that of the L phase, predominantly as a consequence of the influence of the Guinier–Preston–Bagaryatsky (GPB) zone (Cu or Zn segregate) growth in the periphery of the L phase.